Free-space optical modules, such as wavelength-selective switches (WSSs), require a high thermo-mechanical stability. The central wavelength of an optical channel must be maintained within a very narrow wavelength band over a range of operating temperatures, e.g., about 0° C. to about 70° C. Furthermore, insertion loss must be minimized over the range of operating temperatures.
Optical components, such as lenses and prisms, are often formed of materials, such as optical glasses, having a relatively low temperature coefficient of refractive index (dn/dT), in order to reduce temperature-dependent performance degradation. Unfortunately, low-dn/dT materials often have relatively high coefficients of thermal expansion (CTEs).
Typically, optical components in an optical system are joined to a support component, such as an optical bench, by adhesive. The support component is usually formed of a material having a relatively low CTE, in order to lessen thermal expansion of the optical system. The CTE mismatch between the optical components and the support component may lead to high tensile stresses in the optical components, which are often relatively brittle, and in the adhesive joints between the optical components and the support component. In some instances, the CTE mismatch may lead to catastrophic failure, such as fracture of an optical component, upon thermal cycling, shock, or vibration. In other instances, the CTE mismatch may lead to performance degradation, as a result of movement, e.g., tilt, or distortion, e.g., a change in surface curvature, of an optical component.
To alleviate the CTE mismatch, one or more spacers having an intermediate CTE may be disposed between an optical component and a support component, as disclosed in U.S. Pat. No. 6,825,997 to Hubbard, et al., issued on Nov. 30, 2004, and in U.S. Pat. No. 8,292,537 to Newswander, issued on Oct. 23, 2012, for example, which are incorporated herein by reference. In general, the spacers and the components in these disclosures are joined by planar adhesive joints or by fasteners.